NZ213486A

NZ213486A - Continuous vertical casting with axial magnetic field

Info

Publication number: NZ213486A
Application number: NZ213486A
Authority: NZ
Inventors: Charles Vives; Bernard Forest; Jean-Pierre Riquet
Original assignee: Cegedur
Priority date: 1984-09-19
Filing date: 1985-09-16
Publication date: 1988-04-29
Also published as: SG2388G; KR860002326A; FR2570304B1; AU4860485A; NO861808L; SU1473700A3; AU572700B2; BR8506926A; IS1336B6; DE3560766D1; IL76406A0; FR2570304A1; EP0195793B1; FI862091A0; UA5582A1; GB2164280A; NO165581B; FI862091A; PT81155A; GB2164280B

Abstract

In a continuous vertical casting operation carried out in a mould 3, a coil 7 applies a periodic magnetic field of variable strength whose direction is substantially parallel to the axis of the mould 3 to the liquid 5 as it solidifies, in order to regulate the level of the line of contact of the free surface of the metal 5 with the mould 3. The height of the contact of the metal with the mould can be reduced from h1 to h2 by applying the magnetic field, and the strength of the field is adjusted in dependence on the desired level. The procedure is used in casting semifinished metallurgical products, in particular in aluminium and its alloys, in which there is a wish to obtain a cortical region of zero thickness, a fine grain without the preliminary addition of refining agents, and an absence of pitting.

Description

<div class="application article clearfix" id="description"> Priority Date(s): .. J ^7". ff.~. ^ H". Complete Specification Fited: i^?.. Class: ......&2?i&?Z/p.2..- Publication Date: ...»?/?. A??.I???..... P.O. Journal, No: .. 5rfZ NO DRAWINGS NEW ZEALAND PATENTS ACT, 1953 213486 No. Date COMPLETE SPECIFICATION PROCESS FOR REGULATING THE LEVEL OF THE LINE OF CONTACT OF THE FREE 'SURFACE- OF THE METAL WITH THE MOULD IN VERTICAL CASTING We, CEGEDUR SOCIETE DE TRANSFORMATION DE L'ALUMINIUM PECHINEY, a French Company of 23 rue Balzac, 75008 PARIS, France, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is performed, to be particularly described in and by the following statement : -1- (followed by page la) la PROCESS FOR REGULATING THE LEVEL OF THE LINE OF CONTACT OF THE FREE SURFACE OF THE METAL WITH TOE MOULD IN VERTICAL CASTING The invention concerns a process for regulating the level of the line of cot tact of the free surface of the metal with the mould in vertical casting. When manufacturing semifinished metallurgical products by casting ferrous or light metals such as aluminium and alloys thereof, the man skilled in the art seeks to produce ingots, billets, plates etc. which enjoy the best possible decree of physical and" chemical homogeneity, in order to avoid the occurrence of certain defects in the subsequent operation of transforming such products into sheets, wires, etc. Now, most of the casting processes which are used in industry at the present time give rise, when the metal goes from the liquid state to the solid state, to the fonration of homogeneity defects of greater or lesser magnitude, due essentially to the conditions of cooling of the cast products being different from one point to another. Thus, when casting the metal in an ingot mould having a vertical passage in which the metal is successively cooled indirectly by way of the mould and then directly by a sheet of water, it is found that the semifinished products have an external layer referred to as the "primary cortical layer". That layer whose structure and composition differ from those of the internal part of the semifinished products results frorr the ind-rect cooling cf the metal, in contact with the mould. Ir. addition, other forms of heterogeneity which are much less pronounced but which are just as troublesor.e rev appear such as the snail bcles or little pits which are due in particular to cispersion in the rretal rass of the layer of oxide h is at the- surface cf the liquid when in contact with the atmosphere. 213486 A Xi It must be said that the man skilled in the art has not remained inactive when faced with those problems and he has put forward a certain number of solutions which are satisfactory to a greater or lesser degree, seeking to eliminate or at least 5 reduce the seriousness of such heterogenity phenomena. Thus, in U.S. patent No 3 467 166, the man skilled in the art recommended using electromagnetic casting, being a process in which the fact that the metal is confined by means of forces of electromagnetic origin means that it is possible to eliminate the o >,1 ^ 10 mould and thus avoid the appearance of the cortical layer as there is no longer any indirect cooling. That procedure provides for an improvement in the level of homogeneity of the semifinished products. However, that process suffers frcm the following 15 disadvantages: - the casting works must be provided with an electrical installation which is relatively complicated and expensive by virtue of the need to provide currents at a non-industrial frequency (500 to 4000 Hz) in order to generate a suitable confinement field, 20 - the danger of heterogeneity by virtue of small holes or pits is increased by virtue on the one hand of the absence of any mould and therefore an increase in the area of liquid metal which may undergo oxidation, and on the other hand due to the phenomenon of stirring cf the liquid mass which is caused by the confinement field, 25 contributing greatly to dislocation of the film of oxide and dispersion thereof in the metal, - it is often difficult to create a suitable confinement effect v.Ther. starting up the electromagnetic casting process, and - safety of the operating personnel rr-ay be put in quest i 213486 casting aluminium and alloys thereof for, in the event of an electrical failure, as the liquid metal is no longer confined it spreads out outside the mould and can came into contact with the direct cooling fluid, causing an explosion. 5 Other simpler solutions have also been proposed in order to reduce the thickness of the cortical layer. For example, U.S. patent No 3 326 270 teaches using a strip of fiberfrax which is stuck to the mould so as to reduce the height of metal which is in contact with the mould thereby to reduce the effects 10 due to indirect cooling. However, that reduction in height cannot be fixed once and for all as it depends in particular on the casting speed. Thus, when that parameter varies, it is necessary either to change the mould or at least to modify the height of the strip. That means that there is a lack of flexibility in a solution which 15 in the ultimate analysis only provides for partial suppression of the heterogeneity phenomena. In French patent No 1 496 241 (available on request), the disadvantages of indirect cooling are eliminated by using a mould of graphite I which is not cooled, but that process then encounters difficulties in 20 regard to maintenance and frequent changing of the mould, due to the fragility of that material. Another solution is to use moulds which have a striated or ribbed internal surface, by means of which the thickness of the cortical layer is reduced by more than 30% when casting for example 25 aluTtiniuir. 105G. However, besides the machining of such moulds, which considerably increases the cost thereof, that process again suffers from the disadvantages -which arise out of adapting the .-tiojIc and in this case the ribs to each casting speed. * Also known is the process of casting with a feeder head^ o. 3C referred to as the hot. top process, but that also suffers from tra :- - : - 7". .WBfflMWw.w.ii V 4 211 * p *- - *3 '.-. 5 I. disadvantage of giving rise to periodic solidification of the meniscus, being the cause of small folds at the surface of the semifinished products, as well as involving difficulties when starting up the operation. 5 Finally, more recently, New Zealand .patent specification No 189682 claimed a process wherein the axial length of the part of the mould in contact with the liquid metal is varied by using a sleeve which slides on the inside wall surface of the mould. Such a system suffers frccn the disadvantage, upon untimely solidification 10 of the metal, that it causes the mould and the sleeve to stick together, thus causing the components to tear when the sliding movement is started up. It is for that reason that the present applicants, being aware of the problems involved in the above-indicated processes, in 15 an effort to produce homogenous semifinished products in which the thickness of the cortical layer is virtually zero, the product is of a refined grain and the surface of the product is free of pitting, sought and developed a process which has the following advantages over those of the prior art: 20 - the use of electrical installations which are less complicated than those which have to be used for the electromagnetic casting process, - the ease of making the transfer from the casting start-up phase to the steady-state operating phase, 25 - the ease of adaptation to variations in parameters such as the casting speed since the process coes not require any modification ir. the equipment such as a change of mould, - application to any type of conventional mould, - of any arrange-er.t ir; which components are moved, and 2 1348 5 ■•CS V J i 3. H - the risk of an explosion due to liquid metal leakage is less than with the electromagnetic casting process. In order to attain that result, the applicants base themselves on the following observations: 5 - on the one hand, the operation of starting up casting is easier in direct proportion to a rise in the level of metal in the mould. In fact, with a low level, the float which regulates the level and the feed of metal to the mould approaches the solidification front and, for semifinished products of small dimensions, runs the risk 10 of being blocked by untimely solidification of the metal so that_ it can no longer perform its function. Likewise, the camber phenomenon which occurs with semifinished products of substantial width also prevents the casting operation from being started up with the metal at a low level; 15 - on the other hand, in the steady-state phase of operation, it is preferable to cast with the height of metal in the mould being as low as possible, thereby limiting the height of contact of the metal with the wall of the mould and thereby reducing the thickness of the cortical zone which, as has been pointed out above, is 20 essentially due to cooling of the metal by way of the mould. Therefore, taking as the starting point a conventional mould with its various features, that is to say, while retaining a constant height of metal in the mould, since that height is fixed by the position of the float, while being sufficient not to 25 interfere with proper operation of the float, it was therefore necessary to be able to limit to the maximum possible degree the height of contact of the metal with the surface of the mould, which amounted to finding a way of regulating the level of the line of ccr.tact of the free surface of the liquid metal with the wall of the pould. 1 ,/ 2134 8 6 6 According to the invention, the above-indicated way of regulating the level of the line of contact comprises applying to the liquid in the course of solidification, a periodic magnetic field of variable strength and whose direction is substantially 5 parallel to the vertical axis of the mould, and adjusting the strength of the field in dependence on the desired level. a circular coil comprising an electrical circuit formed by one or more windings, and by feeding it with an alternating current at a 10 sufficient industrial voltage, it was possible to modify the profile of the meniscus of the metal and in particular to vary the level of the line of contact of the metal with the nould, more especially as the variations in the feed voltage and in a correlated iranner the strength of the field generated increased. 15 Thus, by increasing the strength of the field, it was possible to lower the level of the line of contact and consequently to reduce the height of the region of contact between the metal and the mould or in contrast, by reducing the strength of the field, it was possible to raise the level of the line of contact and consequently to increase the 20 height of the contact region. makes it possible as desired to reduce the height of the metal- mould contact and consequently the thickness of the cortical layer in a simple fashion with a coil supplied with a current at an industrial 25 electrical supply frequency of 50 or 60 Hz, while being aware that the only repercussion of any electrical failure will be to vary the height of metal in the mould, that is to say, any danger of liquid metal leakage will be eliminated, which is not the case when usinc the electromagnetic casting process. 30 Moreover, the presence of a mould, while limitina the. » o • In fact, it was found that by surrounding the mould with The attraction of such a process is therefore that it _ ...r-i-ff'i'r-.'W'.s I 213486 V ¥ -■i § o possibility of oxidation of the liquid metal at the level of the meniscus, by virtue of the contact of the mould with the metal prevents any displacement of the film of oxide towards the side wall and therefore any danger of pitting at the surface of the 5 semifinished product. Moreover, the field applied to the metal also has the effect of generating forces within the liquid, which homogenise the cooling action and which tend to cause the cast grain to be refined. 10 The shape of the coil which generates the magnetic field is preferably symmetrical with that of the mould so that it generates a field in a direction which is substantially parallel to the vertical axis of the mould. It is disposed along that axis in such a way that the region in which the field exerts its maximum action is at 15 a level of the mould which is between half and a third of the height thereof, as measured from its base. When carrying out a casting operation, such a process makes it possible to provide for normal start-up under the best possible conditions, that is to say, with a high level of metal in 20 the mould. For that purpose, the strength of the field is reduced until it is possibly shut off altogether so as to eliminate any modification in the normal level of the metal. Subsequently, to go into the steady-state operating phase, the strength of the field is increased until a minimum height is reached, resulting in a minimum 25 thickness in the cortical layer. The maximum a±rtissible field strength is easily detected by the occurrence of deformation of the surface of the resulting cast product, when that value is exceeded. It is therefore only necessary to cener-.ine that value in the course of starting up a test casting, anc then use it for ail 30 the casting operations of.the same type. vf E V * itm*1"- -g. — ' •,' ->'4 2i 8 That value generally corresponds to the time at which the level reached by the line of contact corresponds to the level of the line of intersection between the solidification front due to indirect cooling and the soldification front due to direct cooling 5 in a conventional casting operation. The contact height is then virtually reduced to a circular line and the cortical layer does not occur. Depending on the type of alloy being cast, it is known that it will be necessary to carry out the casting operation at 10 different speeds. The process according to the invention makes it possible to modify the strength of the field in order to adapt it to the variations in speed and to determine, as before, the maximum value of the admissible strength of the field for each of those casting speeds. 15 The invention will be better appreciated by reference to the accompanying drawing which is a view in vertical section of two half moulds, that on the left being used in accordance with the prior art while that on the right is used in accordance with the process of the invention. 20 Referring to the drawing, shown therein is a liquid metal feed nozzle 1, a level regulating float 2, and an ingot mould 3 which is cooled directly by the fluid 4 which then cools the metal 5 directly at the point 6. The right-hand half mould is equipped with a coil 7 which is fed with an a.c. voltage 8 in order to generate 25 the iTiagnetic field whose direction is indicated az 9 and to cause a drop in the level of the line of contact of the surface of the metal with the mould frcrr. a point 10 in the prior art casting operation to the point 11 in accordance with the invertion, which i point is cisrosec at the level of the intersection 12 of the V-X30 solidification front 13 resulting fran indirect c:cling and the solidification front 14 resulting from direct cooling. It will be seen therefore that the height of contact of the metal with the mould has been reduced from a value h^ to a value hj which is extremely small arid which can be assimilated to the point 11. The invention may be illustrated by reference to the following examples of use thereof; Taking an aluminium mould with a diameter of 320 rim and a height of 100 rim, an aluminium alloy of type 2214, in accordance with the Aluminium Association standards, was cast at a speed of 60 millimetres per minute. Hie float regulated the level of metal at a position halfway up the mould and the cooling fluid came intd contact with the skin of the cast billet at approximately 1 an below the base of the mould. In a first test, the casting operation was carried out under the conditions of the prior art and micrographic examination of different sections of the billet revealed that the average thickness of the cortical layer was 18 nm. A series of tests was then carried out, in the course of which the mould was surrounded by an annular coil which was 372 mm in inside diameter, 465 nm in outside diameter and 48 irm in height, being formed by 120 turns of enamelled copper wire of a diameter of 3.35 rrm and operated with an alternating current at 50 Hz. Each of the tests was carried out with a different electrical voltage and the corresponding mean cortical thicknesses W$i"€ TTVBSSw red, as well as trie size of the grains, using the intersection methods. The results are set out in the following Table: Voltage in volts 0 50 100 150 180 Cortical thickness in mm 18 16 13 9 0 Grain size in pm 500 300 180 o / . x It is found therefore that the application of the process according to the invention provides a progressive reduction in the 10 thickness of the cortical layer in proportion to an increase in the electrical voltage at the terminals of the coil in proportions such that the thickness of the cortical layer falls to zero at a voltage of 180 volts. At the same time, the grain size decreases so that, 15 starting frcm a metal which has grains measuring 500 ;jm, in conventional casting, the procedure according to the invention gives grains which measure 180 jot on average. Moreover, no pitting is found. The present invention is used in casting semifinished 20 metallurgical products, in particular in aluminium and alloys thereof such as for example lithium alloys and in which there is a wish to produce both a cortical region of virtually zero thickness, 5 fine grain without the preliminary addition of refining agents such as AT53,and the absence of pitting. </div>

Claims

213486 11 WHAT WE CLAIM IS:

1. A process for regulating the level of the line of contact of the free surface of the metal with the mould in a continuous vertical casting operation characterised ^ by applying to the liquid in the course of solidification a periodic magnetic field of variable strength and whose direction is substantially parallel to the vertical axis of the mould and adjusting the strength of the field in dependence on the desired level. o

2. A process according to claim 1 characterised in that the field is at an industrial electrical supply frequency.

3. A process according to claim 1 or claim 2 characterised in that the region in which the field exerts its maximum action is at a level of the mould which is between half and a third of its height measured from its base.

4. A process according to any one of claims 1 to 3 characterised by increasing the strength of the field to lower the level of the line of contact.

5. A process according to any one of claims 1 to 3 characterised by reducing the strength of the field to raise the level of the line of contact.

6. A process according to any one of claims 1 to 3 characterised in that, in a casting operation, the strength of the field is reduced at the time of starting up and it is then progressively increased to a maximum.^%W 12 21 348 value above which the surface of the resulting cast product begins to suffer deformation.

7. A process according to any one of claims 1 to 3 characterised by modifying the strength of the field when the casting speed varies.

8. A process according to claim 1, whenever performed substantially as hereinbefore described with particular reference to the accompanying drawing. DATED THIS DAY OF M.CWC<^ Ml ^ A. J. PARK & SON AGENTS FOR THE APPLICANTS o (1°. k nil <c/